The present invention relates to a cooling apparatus for a gas insulated transformer wherein coils and an iron-core are housed in a tank in which an electrically insulating gas is sealed.
In a conventional oil filled transformer, coils and an iron core are housed in a tank which is filled with an electrically insulating oil. The insulating oil serves to insulate and cool the coils, the iron core and so on. However, use of such an oil filled transformer is not desirable from the viewpoint of safety. A nonflammable transformer is thus desired, an example of which is a gas insulated transformer. In a gas insulated transformer of this type, an electrically insulating gas such as SF.sub.6 gas is sealed in a tank housing coils and an iron core therein to insulate them. The coils and iron core are also cooled upon contact with a volatile cooling medium in the liquid phase.
The cooling medium evaporates by extracting heat from the coils. The vapor of the cooling medium is coexistent with a noncondensable insulating gas in the tank. If the noncondensable insulating gas is mixed in the vapor of the cooling medium, even in a small amount, condensation heat transfer coefficient of the vapor of the cooling medium is significantly lowered. In a gas insulated transformer of the vaporization cooled type which is cooled with the cooling medium, the coils are cooled by being sprayed with the cooling medium in the liquid phase, and the vapor of the cooling medium generated by this cooling process is condensed by a cooling unit. However, as mentioned earlier, mixing of the insulating gas renders condensation of the vapor of the cooling medium difficult. For this reason, the temperature of the cooling medium for cooling the coils and the like is raised, resulting in degradation of the cooling efficiency and an increase in the internal pressure of the tank. In order to prevent these problems, a cooling unit of large capacity must be mounted, and the overall apparatus becomes bulky, costly and heavy.
A gas insulated transformer, of separate cooling type has also been proposed wherein a duct for flowing a cooling medium therethrough is incorporated within the coils so as to cool the coils, instead of spraying them with the cooling medium. However, in order to obtain satisfactory cooling effects, a cooling duct of small wall thickness must be incorporated throughout the height (about 1 to 2 m) of the coils, which makes the manufacture of the apparatus difficult.